LED lighting devices are known for their high efficiency and longevity. However LED lamps are complex solid-state devices which can easily be damaged when operated outside of their permissible limits. In addition there is no known existing system for economically and accurately controlling performance characteristics or parameters of the LED light source including longevity, intensity and color.
One problem occurs because LEDs change resistance as the temperature of their PN junction changes. The variable resistance of the LED junction makes prevention of overheating or damage to the LED difficult. The variable resistance of the LED junction makes operation of the LED at a constant intensity, color, longevity or efficiency difficult.
Also, LEDs are prone to damage if the power delivered by the source of electrical power exceeds the respective power limitation for the LED. Adding to the complexity of LEDs is the fact that a maximum power limitation for the LED is not always a fixed value. The maximum power limitation can change with a number of parameters including ambient temperature, thermal circuit resistance, etc.
LED lamps are typically mounted on a circuit board and within a fixture. The resulting assembly provides a thermal path (thermal circuit) for the heat to be removed from the PN junction. If the thermal circuit is inadequate in that the thermal circuit is incapable of removing the necessary amount of heat from the PN junction—or if the thermal circuit changes such that the thermal circuit becomes inadequate, the PN junction can overheat and be damaged. As a consequence, circuits that are economical and commercial and effectively protect the PN junction require an excessively large safety factor.
A first design adjusts the magnitude of a constant current circuit to counter increases in the ambient temperature to prevent damaging the LED. This design adds a thermally sensitive resistor (thermistor) to the circuit to interact with the current control device. An example of this design can be found in Linear Technology Corp design note 388, FIG. 6, in which auxiliary components are added to a constant current control circuit LT 3474. The thermally sensitive resistor is typically mounted on the printed circuit board close to the LED and, upon an increase in ambient temperature, sends a signal to the current control device to change the value of constant current to a new value which is appropriate for the existing ambient temperature. This system is an improvement over the basic constant current circuit as it does initiate actions to protect the LED from overheating. However, the system is limited in that there is a time lag-due to the separation between the PN junction of the LED and the thermistor—before the thermally sensitive resistor sends a signal to the current control device. In addition, the thermally sensitive resistor is not positioned precisely at the PN junction. Therefore, the system does not experience the same temperature change as the PN junction. Hence, the thermally sensitive resistor's signal to the current control device does not fully address the ambient temperature problem.
Another design for an LED control utilizing dynamic resistance of LEDs utilizes the dynamic resistance of an array of LEDs to maintain the current through the LEDs at a desired level. This dynamic resistance design is similar to the constant current circuit in which an internal current sensing resistor of a constant current microchip circuit in a series arrangement with an LED array responds to a change in its current (also the current supplied to the LED array) by creating a signal. The signal adjusts the current flowing into the load or LED array to counter the changing current and return the sensed value of the current to the constant current level. Therefore, the dynamic resistance design does not prevent the LED from being damaged by overheating due to increases in the ambient temperature. The dynamic resistance design is limited in that the design requires an array of LED lamps with a dynamic resistance matched to the current control circuit. Finally, the dynamic resistance design does not maintain the performance parameters such as intensity, color, longevity etc. at target levels throughout changes in uncontrolled ambient conditions.
Thus, a need exists for effectively protecting the LED from damage and for the ability to maintain constant performance parameters such as intensity, color, longevity etc. of the emitted light.